Lithium secondary batteries have such characteristics as a high energy density and a long life span. Therefore, lithium secondary batteries are used widely as power sources for home appliances such as video cameras and portable electronic devices such as notebook personal computers and mobile phones, electric tools such as power tools, and the like, and recently have been put into application in large batteries that equip an electric vehicle (EV), a hybrid electric vehicle (HEV) and the like.
A lithium secondary battery is a secondary battery having a structure in which, during charging, lithium melts out from the positive electrode as an ion and moves towards the negative electrode to be stored and conversely, during discharging, the lithium ion returns from the negative electrode to the positive electrode, and it is known that the source of the high energy density of the battery lies in the electric potential of the positive electrode material.
Known as positive electrode active materials for lithium secondary batteries of this species, are spinel-type composite oxides containing lithium and manganese having a spinel structure (Fd-3m) of the manganese series, such as LiMn2O4, and LiNi0.5Mn1.5O4, in addition to lithium transition metal oxides such as LiCoO2, LiNiO2 and LiMnO2 having a layer structure.
Owing to low raw material costs and the absence of toxicity, which renders it safe, and further more, having the property of being strong against over-charging, there is a focus on this species of spinel-type lithium-manganese composite oxide as a next-generation positive electrode active material for use in a large battery for an electric vehicle (EV), a hybrid electric vehicle (HEV) and the like. In addition, a spinel-type lithium transition metal oxide (LMO), which allows for insertion and desorption of Li ions three-dimensionally, has excellent output characteristics, compared to a lithium transition metal oxide such as LiCoO2, which has a layer structure, such that utilization in applications requiring excellent output characteristics such as EV batteries, HEV batteries, and the like, is anticipated.
Above all, it is now known that substituting a portion of the Mn sites in LiMn2O4 with another transition metal (Cr, Co, Ni, Fe or Cu) gives an operating electric potential of close to 5 V, and a considerable amount of development is currently under way for a manganese series spinel-type lithium transition metal oxide having an operating electric potential of 4.5 V or higher (5 V-class).
For instance, as a positive electrode active substance for a lithium secondary battery exhibiting a 5 V-class electromotive force, Patent Document 1 describes a high-capacity spinel-type lithium-manganese composite oxide positive electrode active substance, comprising a spinel-type lithium-manganese composite oxide added with chromium as an essential additive component, and further, nickel or cobalt.
Patent Document 2 describes a crystal LiMn2−y−zNiyMzO4 (where M represents at least one species chosen from the group comprising Fe, Co, Ti, V, Mg, Zn, Ga, Nb, Mo and Cu; and 0.25≦y≦0.6, 0≦z≦0.1) having a spinel structure carrying out charge-discharging with an electric potential of 4.5 V or higher relative to Li metal.
As a positive electrode material for a high energy density lithium ion secondary battery having a high voltage of 4.5 V or higher relative to Li, Patent Document 3 describes a spinel-type lithium-manganese composite oxide represented by Lia(MxMn2−x−yAy)O4 (where 0.4<x, 0<y, x+y<2, 0<a<1.2; and M contains one or more species of metal elements chosen from the group comprising Ni, Co, Fe, Cr and Cu, and contains at least Ni; A contains at least one species of metal element chosen from Si and Ti, with the value of y, which is the ratio for A, being 0.1<y, when A only contains Ti).
As a positive electrode active substance whereby the capacity density becomes high owing to the tap density of the positive electrode active substance and the initial discharge capacity of a secondary battery using this positive electrode active substance being both high, Patent Document 4 describes a lithium nickel manganese composite oxide having a spinel structure represented by formula (I): Li1+xNi0.5−1/4x−1/4yMn1.5−3/4x−3/4yByO4 (in formula (I), x and y satisfy 0≦x≦0.025 and 0<y≦0.01); the lithium nickel manganese composite oxide having a median diameter of 5 to 20μ, a particle size variation coefficient of 2.0 to 3.5%, and a BET specific surface area of 0.30 to 1.30 m/g.
A problem exists, that when lithium nickel manganese composite oxides having a spinel structure are used as positive electrode active substances in lithium secondary batteries, sometimes, the electrolytic solution decomposes and generates gas. Among them, for manganese series spinel-type lithium transition metal oxides having an operating electric potential of 4.5 V or higher (5 V-class), it is a crucial problem that should be solved in particular.
As a cause of such gas generation, a prior art thought is that an impurity contained in a positive electrode active substance reacts with the electrolytic solution to generate the gas, which has led to the method of removing water-soluble impurities by washing with water being proposed.
For instance, Patent Document 5 describes a production method for a positive electrode active substance for a non-aqueous electrolytic solution secondary battery, wherein a lithium compound, a manganese compound, and at least one species of metal or metal compound chosen from the group comprising Ni, Al, Co, Fe, Mg and Ca are mixed and fired to obtain a lithium manganese oxide, then, this lithium manganese oxide is washed with water and then filtered and dried, thereby obtaining the positive electrode active substance for a non-aqueous electrolytic solution secondary battery.
Elsewhere, Patent Documents 6, 7, 8, and the like, also describe methods for removing impurities on a particle surface by water-washing a spinel-type lithium transition metal oxide obtained by firing.